An isolation technology for separating transmitting/receiving signals in an antenna has been studied for a long time. The conventional isolation technology can be divided into two technologies according to propagation direction.
The first one is a unidirectional repeater, i.e., an antenna technology for receiving a receiving signal in the rear part and transmitting an amplified signal in the front part. This technology is used in a repeater system for co-channel broadcasting and it will be called a unidirectional isolation technology hereinafter. In the antenna technology, the transmitting signal is not transmitted in a direction that the transmitting signal is received, and a repeater for the co-channel broadcasting is used representatively and generally.
Since the unidirectional isolation technology gains high isolation by setting up high-directional antennas in opposition to each other and spacing them from each other, the unidirectional isolation technology has a shortcoming that it requires much space for setup.
Therefore, a broadcasting repeater using the conventional unidirectional isolation technology is used by setting up a receiving antenna in the lower part of a high iron tower and a transmitting antenna in the upper part.
The other conventional technology is a bi-directional isolation technology for re-transmitting in a co-direction. Specifically, there is a bi-directional isolation technology based on polarization.
That is, the technology raises the degree of isolation of the transmitting/receiving signals by generating polarizations of the transmitting/receiving signals.
However, the conventional bi-directional isolation technology has a problem that a distance length should be sufficiently acquired between the receiving antenna, i.e., the horizontal polarization and the transmitting antenna, i.e., the vertical polarization.
Another conventional technology, which is not applied to a practical system, generates the transmitting signal and the receiving signal, whose polarizations are perpendicular to each other, and maintains isolation between two terminals by vertically setting up their feed in a patch antenna. This is revealed in an article by Karode, in IEE National Conference on Antennas and Propagation, pp. 49-52, April 1999).
Also, Hao has realized an isolation technology by differentiating polarization generation of the patch antenna applying a photo band gap (PBG) structure in an article published in IEE, 11th International Conference on Antenna Propagation, pp. 86-89, April 2001).
However, as shown in the result, since the isolation for a co-frequency of the transmitting/receiving signals is very low, the technology is not proper as an antenna for co-channel bi-directional communications in diverse fields of mobile communications, short distance communications, a broadcasting repeater and satellite communications requiring high isolation for the co-frequency.
In the result of the conventional technologies suggested by Karodo and Hao, isolation is not more than about 60 dB although transmitting/receiving frequency bands or polarizations are different from each other.
Therefore, the isolation antenna, which is required for a co-channel wireless Local Area Network (LAN) low output repeater, a Radio Frequency (RF) reader antenna, an antenna for testing Radar Cross Section (RCS), and an isolation antenna capable of co-channel, co-polarization and bi-directional communications which is necessary in co-channel bi-directional communication devices.